Internal combustion engines convert chemical energy in a fuel to mechanical energy. As part of the conversion, the fuel can be combusted, thus causing hot combustion products to expand within the engine. The expansion of the combustion products can be used to move mechanical components of the engine, such as pistons. Combustion reactions can have several products, or emissions, some of which can be undesirable. For example, when hydrocarbons are used as fuel, combustion products can include HC, CO, CO2 and NOx.
An internal combustion engine may operate in one or more combustion modes. One example mode is spark ignition (SI), where an electric spark from a sparking device is used to initiate combustion of an air and fuel mixture. Another example mode is homogeneous charge compression ignition (HCCI), where an air and fuel mixture achieves a temperature where autoignition occurs without requiring a spark from a sparking device. In some conditions, HCCI may have greater fuel efficiency, reduced NOx production, and/or other advantages compared to SI. However, in some conditions, such as with high or low engine loads and/or high or low engine speeds, it may be difficult to achieve reliable HCCI combustion.
Numerous attempts have been made to design a dual combustion mode engine that is configured to utilize SI during some conditions and HCCI during other conditions. For example, U.S. Pat. No. 6,619,254 describes a dual combustion mode engine that uses SI and HCCI. Further, a third combustion mode is described where under certain operating conditions the pressure, temperature, and composition of the charge are set in such a way that the self-ignition capability is just short of being reached, and an external energy source in the form of an electric spark or an additionally injected quantity of fuel is used to trigger ignition.
The inventors herein have recognized disadvantages with previous attempts at HCCI operating mode engines, including dual mode engines that use SI combustion at least some of the time. Since SI combustion is generally hotter than HCCI combustion, when switching from SI operation to HCCI operation, there is a period when the temperature of combustion is decreasing with each combustion event and a hybrid SI-HCCI combustion occurs. Similarly, when switching back into SI operation, the temperature of combustion is expected to increase back up to SI levels during the hybrid combustion phase. This hybrid combustion is suboptimal in terms of stability, efficiency, and emissions generation. If not well controlled, such a hybrid combustion can cause misfire, and in an extreme case, combustion can cease altogether.
Furthermore, if a spark assist is used during transition periods between SI and HCCI operation, some of the benefits of HCCI may not be fully realized. In particular, NOx emissions and/or fuel economy may be less favorable than if HCCI was used without spark assist. Thus, it may be advantageous to reduce transitions between combustion modes by using a heating device to facilitate HCCI, at least during some conditions. In this manner, the HCCI operating region may be expanded and/or HCCI operation can be maintained during changes in engine operating conditions. In this way, it may be possible to decrease SI or hybrid SI/HCCI in favor of HCCI, thus further realizing the benefits of HCCI.